1. Field of the Invention
The present invention relates to a process for producing titanium dioxide by oxidizing titanium tetrachloride in the gaseous phase at high temperature.
2. Description of the Prior Art
In the preparation of titanium dioxide having suitable characteristics for use as a pigment by the gaseous oxidation of titanium tetrachloride, the following conditions are required:
1. The temperature of the titanium tetrachloride and oxygen in the reaction zone should be high enough to produce the oxidizing reaction.
2. The time from the initiation of the reaction to its finish should be made as short as possible. In the continuous reaction of gaseous titanium tetrachloride and oxygen, the reaction zone should be kept as small as possible.
3. The product should be rapidly and uniformly cooled after the finish of the reaction.
In the practice of the preparation of titanium dioxide by gaseous oxidation, several difficulties are known to occur. When the reaction zone is minimized by raising the temperature of the pre-mixed raw material gases in order to satisfy conditions (1) and (2), the resulting titanium dioxide is deposited in the burner, or on the inner wall of the reactor, thereby closing the passage to the reactor. Moreover, the deposited particles of titanium dioxide become rough whereby they are not suitable for use as a pigment. Regarding condition (3), when the resulting titanium dioxide is not rapidly and uniformly cooled after the oxidation to lower than 1000.degree.C, preferably lower than 800.degree.C, the resulting titanium dioxide particles become enlarged whereby they are not suitable for use as a pigment. Heretofore, in order to overcome these difficulties, there has been proposed the employment of a method of utilizing a down-flowing cascade of incandescent rough particles of titanium dioxide while burning titanium tetrachloride with oxygen as a jet current in a substantially vertical, upward direction, in a reaction zone surrounded by said down-flow; or a method of impinging fine particles on a reactor wall that is accessible to the reactants and is adjacent to the raw material gas inlet of the reactor. However, the following disadvantages have been found in the known methods:
In the former method, some of the titanium dioxide resulting from the reaction adheres to the surface of the rough particles of the down-flowing titanium dioxide, forming a coating thereupon, whereby the yield of the reaction product of titanium dioxide is decreased. Accordingly, it is necessary to recover the titanium dioxide from the rough particles having such coatings by recycling them through a chlorination step. In this method, the reaction product is excessively cooled by suspending the fine particles in a carrier gas. This tends to slow the reaction. Also, since the fine particles impinge upon a portion of the reactor which is adjacent to the raw material gas inlet, damage or abrasion of the raw material gas inlet may result, thereby requiring substantial apparatus maintenance. In order to rapidly cool the resulting titanium dioxide, several methods have been proposed: indirectly cooling by heat exchange on a cool surface; introduction of cool inert particles to rapidly cool the product followed by separation from the product and recycling, and use of an inert cooling gas. The latter method of using an inert cooling gas has special advantages in that it requires no special apparatus and is effective with a low failure rate. However, these methods have the disadvantages of requiring an increased volume of gas and requiring an increase in the size of the apparatus in order to accommodate the cooling and separating steps. It would be desirable to provide a cooling method which would permit a substantial reduction in volume of cooling gas required.